Thin-film solar cells

ABSTRACT

A method of manufacturing improved thin-film solar cells entirely by sputtering includes a high efficiency back contact/reflecting multi-layer containing at least one barrier layer consisting of a transition metal nitride. A copper indium gallium diselenide (Cu(In X Ga 1−X )Se 2 ) absorber layer (X ranging from 1 to approximately 0.7) is co-sputtered from specially prepared electrically conductive targets using dual cylindrical rotary magnetron technology. The band gap of the absorber layer can be graded by varying the gallium content, and by replacing the gallium partially or totally with aluminum. Alternately the absorber layer is reactively sputtered from metal alloy targets in the presence of hydrogen selenide gas. RF sputtering is used to deposit a non-cadmium containing window layer of ZnS. The top transparent electrode is reactively sputtered aluminum doped ZnO. A unique modular vacuum roll-to-roll sputtering machine is described. The machine is adapted to incorporate dual cylindrical rotary magnetron technology to manufacture the improved solar cell material in a single pass.

This application claims the benefit of U.S. Provisional Application No.60/415,009, filed Sep. 30, 2002; and of U.S. Provisional Application No.60/435,814, filed Dec. 19, 2002.

FIELD OF THE INVENTION

The invention disclosed herein relates generally to the field ofphotovoltaics, and more specifically to a unique high throughputroll-to-roll vacuum deposition system and method for the manufacturingof thin-film solar cells based upon absorbing layers that containcopper, indium, gallium, aluminum, and selenium and have apolycrystalline chalcopyrite structure.

BACKGROUND OF THE INVENTION

Interest in thin-film photovoltaics has expanded in recent years. Thisis due primarily to improvements in conversion efficiency of cells madeat the laboratory scale, and the anticipation that manufacturing costscan be significantly reduced compared to the older and more expensivecrystalline and polycrystalline silicon technology. The term “thin-film”is used to distinguish this type of solar cell from the more commonsilicon based cell, which uses a relatively thick silicon wafer. Whilesingle crystal silicon cells still hold the record for conversionefficiency at over 20%, thin-film cells have been produced which performclose to this level. Therefore, performance of the thin-film cells is nolonger the major issue that limits their commercial use. The mostimportant factor now driving the commercialization of thin-film solarcells is cost. Currently, a widely accepted technology solution for thescale up to low-cost manufacturing does not exist.

Attempts have been made and are now being made to remedy the problem,but progress has been slow. While a large infrastructure exists for thesputter coating of glass for the architectural window market, thisprocess is not readily adapted to the production of solar cells forseveral reasons. First, the glass that is coated in large-scale machinesis relatively thick compared to that used in solar modules. Also, theglass must be heated to temperatures far above that required in thewindow industry, causing large yield losses from fracturing andbreakage. Handling large sheets of glass is expensive in terms of floorspace and equipment, and the extra layers in a solar cell requireadditional large coating chambers with appropriate gas isolation betweenchambers. Finally, and maybe most importantly, efficient sputteringtargets have not yet been made for the deposition of the absorber layer,which in many respects is the most challenging aspect of making athin-film solar cell.

An early attempt to improve manufacturing of solar cells with aroll-to-roll technique was proposed by Barnett et al in U.S. Pat. No.4,318,938 ('938) issued 9 Mar. 1982. They describe a roll-to-rollmachine, which consists essentially of a series of individual batchprocessing chambers each adapted to the formation of a different layer.A thin foil substrate is continuously fed from a roll in a linearbelt-like fashion through the series of individual chambers where itreceives the required layers. Several of the layers are formed byevaporation of the desired material in vacuum chambers. The metal foilis transferred continuously from air to vacuum and back to air severaltimes. The patent does not describe how this is accomplished, other thanthe statement that such technology can be purchased. Much has changed inrecent years. The copper sulfide absorber layer proposed in '938 hasbeen shown to be unstable in the field, and some of the other layers areno longer used. In particular, it is undesirable to have a pinch rollerrunning on a newly formed coating layer. However, the inventorsestimated that their continuous technique could reduce the manufacturingcost by as much as a factor of two over the conventional batch processfor silicon. While a factor of two is still significant today, greaterreductions in cost must be achieved if solar power is to becomecompetitive with conventional sources of power generation.

Matsuda et al in U.S. Pat. No. 5,571,749 ('749) issued 5 Nov. 1996 teacha roll-to-roll coating system based on plasma chemical vapor deposition(CVD) techniques. Their system is a single linear vacuum chamber with aseries of six gas gates for process isolation. The web substrate ispassed through the machine in belt-like fashion similar to the method of'938, but the web remains in vacuum for the whole process. The solarcell absorbing layer is made from amorphous silicon deposited from thedecomposition of silane gas. Different dopants are introduced along thebelt path to create the required p-n junctions. Similar techniques areused at Uni-Solar of Troy, Mich. to make a variety of amorphous siliconsolar cells. The conversion efficiency of amorphous silicon cells isinferior to that of the other thin film cells, and they suffer a loss ofefficiency during the initial few weeks of exposure to solar radiationthrough a mechanism known as the Stabler-Wronski effect. Because of thisthe efficiencies of amorphous silicon remain well below that of otherthin-film materials, and no one has yet found a way to mitigate theeffect.

Wendt et al disclose a roll-to-roll system in U.S. Pat. No. 6,372,538('538) issued 16 Apr. 2002 that teaches a method for depositing a thinfilm solar cell based upon a copper indium/gallium diselenide (CIGS)absorber layer. The system is described as consisting of nine separateindividual processing chambers in which a roll-to-roll process may beused at each chamber. Thus the overall system is similar to thatdescribed in '938, but without the continuous belt-like transport of thesubstrate through all of the chambers at once. Also the roll of thinmaterial (polyimide in this case) is not continuously fed through asingle vacuum system as it is in '749. Wendt et al teach conventionalplanar magnetron sputtering for the deposition of a molybdenum backcontact layer onto the polyimide film. Adjustments are made to the argongas pressure and some oxygen is introduced to adjust the film stress toaccommodate the expansion of the polyimide when it is heated for theCIGS deposition. Incorporation of oxygen into the molybdenum layerincreases its resistivity, requiring the layer to be thicker to provideadequate electrical conductivity. The CIGS materials are deposited overthe molybdenum layer in a separate chamber using an array of thermalevaporators each depositing one of the components. The use of thepolyimide substrate material presents at least two problems inprocessing. First, it contains a relatively large amount of adsorbedwater, which is evolved in the vacuum system and can have negativeeffects on the process. And secondly, it cannot withstand the highertemperatures used for the deposition of high quality CIGS material. Thinfoils of stainless steel would have neither of these problems. Thepreferred width of the polyimide web is 33 cm, and it runs at a typicalline speed of 30 cm per minute. With respect to the present invention,such production rates (about a square foot per minute) are notconsidered large-scale; rather, rates 5 to 10 times faster withattendant cost reductions are necessary to make solar power competitivewith power from conventional sources.

Copper indium diselenide (CuInSe₂ or CIS) and its higher band gapvariants copper indium gallium diselenide (Cu(In/Ga)Se₂ or CIGS), copperindium aluminum diselenide (Cu(In/Al)Se₂), and any of these compoundswith sulfur replacing some of the selenium represent a group ofmaterials that have desirable properties for use as the absorber layerin thin-film solar cells. The acronyms CIS and CIGS have been in commonuse in the literature for sometime. The aluminum bearing variants haveno common acronym as yet, so CIGS is used here in an expanded sense torepresent the entire group of CIS based alloys. To function as a solarabsorber layer these materials must be p-type semiconductors. This isaccomplished by establishing a slight deficiency in copper, whilemaintaining a chalcopyrite crystalline structure. Gallium usuallyreplaces 20% to 30% of the normal indium content to raise the band gap;however, there are significant and useful variations outside of thisrange. If gallium is replaced by aluminum, smaller amounts of aluminumare required to achieve the same band gap.

CIGS thin-film solar cells are normally produced by first depositing amolybdenum (moly) base electrical contact layer onto a substrate such asglass, stainless steel foil, or other functional substrate material. Arelatively thick layer of CIGS is then deposited on the moly layer byone of two widely used techniques. In the precursor technique, themetals (Cu/In/Ga) are first deposited onto the substrate using aphysical vapor deposition (PVD) process (i.e. evaporation orsputtering), chemical bath, or electroplating process. Subsequently, aselenium bearing gas is reacted with the metals layer in a diffusionfurnace at temperatures ranging up to about 600° C. to form the finalCIGS composition. The most commonly used selenium bearing gas ishydrogen selenide, which is extremely toxic to humans and requires greatcare in its use. A second technique avoids the use of hydrogen selenidegas by co-evaporating all of the CIGS constituents onto a hot substratefrom separate thermal evaporation sources. While the deposition ratesare relatively high for thermal evaporation, the sources are difficultto control to achieve both the required stoichiometry and thicknessuniformity over large areas of a substrate. Neither of these techniquesfor forming the CIGS layer is readily scalable to efficient large-scaleproduction.

In part, moly is used as the back contact layer because of the hightemperature required for the CIGS deposition. Other metals (silver,aluminum, copper etc.) tend to diffuse into and/or react with theselenium in the CIGS at the elevated deposition temperatures, and createan undesirable doping or interface between the contact layer and theCIGS layer. Moly has a very high melting point (2610 C), which helps toavoid this problem, although it will react with selenium at hightemperatures. However, even if the reactive interface is minimized, molystill has a rather poor reflection at the interface with the CIGS layer,resulting in decreased efficiency since the light that penetrates theabsorber initially is not reflected back through the CIGS effectivelyfor a second chance at being absorbed. Therefore, replacing the molywith a better reflecting layer can allow a decrease in the thickness ofthe absorber layer as well as provide improved cell performance bymoving the absorption events closer to the p-n junction.

The n-type material most often used with CIGS absorbers to form the thin“window” or “buffer” layer is cadmium sulfide (CdS). It is much thinnerthan the CIGS layer and is usually applied by chemical bath deposition(CBD). Cadmium is toxic and the chemical bath waste poses anenvironmental disposal problem, adding to the expense of manufacturingthe cell. CBD zinc sulfide (ZnS) has been used successfully as asubstitute for CdS, and has produced cells of comparable quality. TheCBD method for ZnS is not as toxic as CdS; but, remains a relativelyexpensive and time-consuming process step, which should be avoided ifpossible. Radio frequency (RF) sputter deposition of CdS and ZnS hasbeen demonstrated on a small scale. However, RF sputtering over largeareas is difficult to control because the plasma is highly influenced bythe chamber geometry in the conventional method of implementing RFsputtering. An improved method of RF sputtering ZnS is needed to reducethe process complexity as well as to eliminate the toxic cadmium fromthe process.

Finally, the window or buffer layer is covered with a relatively thicktransparent electrically conducting oxide, which is also an n-typesemiconductor. In the past zinc oxide (ZnO) has been used as analternative to the traditional, but more expensive, indium tin oxide(ITO). Recently, aluminum doped ZnO has been shown to perform about aswell as ITO, and it has become the material of choice in the industry. Athin “intrinsic” (meaning highly resistive) ZnO layer is often depositedon top of the buffer layer to cover any plating flaws in the CdS (hence“buffer” layer) before the cell is completed by the deposition of thetransparent top conductive layer. In order to further optimize theperformance of the cell, an antireflection coating may be applied as afinal step. Because of differences in refractive index, this step ismore important for silicon cells than for CIGS cells in which some levelof antireflection is provided by the encapsulation material when thecells are made into modules. In the case of CIGS an antireflectioncoating may be applied to the outer surface of the glass.

The difficulties inherent in the deposition of CIGS related absorberlayers as well as the buffer layer have prevented these thin-film solarcells from being readily manufactured in large scale with improvedeconomies and lower costs. Concurrent improvements in the back reflectorand elimination of cadmium and its waste disposal problems can alsolower the cost per watt of generated solar power.

A conventional prior art CIGS solar cell structure is shown in FIG. 1.Because of the large range in the thickness of the different layers,they are depicted schematically. The materials most often used for eachof the layers are also indicated in the figure. The arrow at the top ofthe figure shows the direction of the solar illumination on the cell.Element 1 is the substrate, and it is massive in relation to thethin-film layers that are deposited upon it. Glass is the substrate thathas been commonly used in solar cell research; however, it is morelikely that for large-scale production some type of foil-like substratewill be used. Layer 2 is the back electrical contact for the cell.Traditionally, it has been moly with a thickness of about 0.5 to 1.0microns. While moly has been shown to be compatible with CIGS chemistryand the relatively high temperature of the CIGS deposition, it has somedisadvantages. It is more expensive than other metals that are betterconductors (aluminum or copper for example), and it is not a goodreflector in the spectral region of the maximum solar output. Thus lightthat does not create electron-hole pairs in the CIGS absorber on itsfirst transit is not efficiently reflected back through the absorber fora second chance at causing a photoelectric event. Light that is absorbedin the moly, including the part of the solar spectrum that falls outsideof the CIGS absorption band, only contributes to heating of the cell,which lowers its overall conversion efficiency. A better back electrodematerial is desirable in a large-scale manufacturing system.

Layer 3 is the CIGS p-type semiconductor absorber layer. It is usuallyabout 2 to 3 microns thick, but could be somewhat thinner and attain thesame or improved efficiency if the reflection of the back electrodelayer (2) were improved. It would be extremely desirable to produce thislayer by magnetron sputtering. This would enable a large-scalemanufacturing process because magnetrons can readily be made in largesizes, and thickness and composition control can be excellent. A majorprovision of this invention is to demonstrate how this can be done withCIGS materials. Layer 4 is the n-type semiconductor layer that completesthe formation of the p-n junction. It is much thinner than the absorberlayer (about 0.05 microns), and it should be highly transparent to thesolar radiation. Traditionally, it has been called the window layer,since it lets the light pass down to the absorber layer. It is alsoreferred to as a buffer layer because it seems to help protect the p-njunction from damage induced by the deposition of the next layer. So farthe use of CdS has resulted in the highest efficiency cells for the CIGStype absorber materials. But CdS is environmentally toxic, and it isdifficult to deposit uniformly in large-scale by either the chemicalbath method or by conventional RF magnetron sputtering. In addition, CdSis not highly transparent to the green and blue region of the solarspectrum, which makes it less compatible with higher band gap absorberlayers.

At the 26^(th) IEEE Photovoltaic Specialists Conference in October of1977 Ullal, Zweibel, and von Roedern suggested a list of fifteennon-cadmium containing n-type materials that might be used assubstitutes for the CdS layer. Of those materials SnO₂, ZnO, ZrO₂, anddoped ZnO, are readily deposited by ordinary reactive magnetronsputtering of the metal in an argon and oxygen atmosphere. The reactivesputtering method that uses dual cylindrical rotary magnetrons as taughtin U.S. Pat. No. 6,365,010 ('010) is especially useful for depositingthese oxide layers. However, the dual cylindrical rotary magnetrontechnology can easily be extended to the reactive sputtering of sulfidesand selenides, if facility improvements are made to handle the deliveryof small amounts of the hydrogen sulfide and hydrogen selenide gases tothe reactive deposition region. Using this technique two of the othermaterials on the list, ZnS and ZnSe, can be readily deposited with thedual cylindrical rotary magnetron system in the reactive mode. ZnSdeposited by other methods has already been used instead of CdS in alaboratory demonstration cell that achieved a conversion efficiency of18%. In addition, both ZnS and ZnSe have larger band gaps than CdS, sothey are more efficient window materials. The less desirable method ofconventional RF sputtering would work marginally for depositing thinlayers of any remaining materials that cannot be readily formed intoconducting targets.

Layer 5 is the top transparent electrode, which completes thefunctioning cell. This layer needs to be both highly conductive and astransparent as possible to solar radiation. ZnO has been the traditionalmaterial used with CIGS, but indium tin oxide (ITO), Al doped ZnO, and afew other materials could perform as well. Layer 6 is the antireflection(AR) coating, which can allow a significant amount of extra light intothe cell. Depending on the intended use of the cell, it might bedeposited directly on the top conductor (as illustrated), or on aseparate cover glass, or both. For space-based power it is desirable toeliminate the cover glass, which adds significantly to expensive launchweight. Ideally, the AR coating would reduce the reflection of the cellto very near zero over the spectral region that photoelectric absorptionoccurs, and at the same time increase the reflection in the otherspectral regions to reduce heating. Simple AR coatings do not adequatelycover the relatively broad spectral absorption region of a solar cell,so multiple layer designs that are more expensive must be used to do thejob more efficiently. Coatings that both perform the AR function andincrease the reflection of unwanted radiation require even more layersand significant coating system sophistication. Aguilera et al in U.S.Pat. No. 6,107,564 issued 22 Aug. 2000 thoroughly review the prior art,and offer some improved AR coating designs for solar cell covers.

As previously mentioned the moly back contact layer is not a goodreflector, nevertheless it has become the standard for thin-film typesolar cells. Finding a better reflecting material that would otherwisewithstand the processing conditions could improve the cell performance.The task is not simple. The back layer simultaneously should be a goodconductor, be able to withstand high processing temperatures, and itshould be a good reflector. Many metals in the periodic table meet atleast one of these requirements, and any metal could be made thickenough to provide enough conductivity to function as the back electricalcontact. The requirement of high processing temperatures eliminates thelow melting point metals from consideration. Metals like tin, lead,indium, zinc, bismuth, and a few others melt at temperatures below theprocessing temperature for the CIGS or most other solar absorbermaterials. The motivation to lower the cost of the cell excludes metalslike gold, platinum, palladium, rhodium, ruthenium, iridium, and osmiumwhich otherwise have good conduction and reasonable reflectionproperties. With the exception of magnesium, which is highly reactive,all of the rest of the metals on the left half of the periodic chart ofthe elements are relatively poor reflectors, including molybdenum. Theremaining candidates include aluminum, copper, silver, and nickel, andonly nickel (and to a lesser extent molybdenum) resists forminginsulating and poorly reflecting selenium compounds at the CIGSinterface. However, nickel will severely degrade CIGS material if it isallowed to diffuse into it.

It is desirable to improve the large-scale manufacturability ofthin-film solar cells in order to reduce their cost and make themcompetitive with conventional sources of electrical power generation.The use of the term large-scale in the context of the present inventionimplies the coating of either discrete substrates or continuous websthat have width dimensions of about a meter or more. This inventionprovides an apparatus and a method for sputter depositing all of thelayers in the solar cell, and particularly the CIGS layer, which greatlyincreases the deposition area over which the required properties of thematerial can be achieved and controlled. It also provides improvementsto the back contact/reflecting layer and the elimination of cadmium fromthe process.

SUMMARY OF THE INVENTION

An approach to solving problems with conventional CIGS solar cells ispresented by Iwasaki et al in U.S. Pat. No. 5,986,204 ('204). Theyconsider the same list of candidate metals that were just discussedabove; however, they propose using alloys of silver-aluminum andcopper-aluminum for the back conductor. A limitation to use of thesealloys is that they must be applied at relatively low processtemperatures, below about 120 C, which are marginally appropriate foramorphous silicon absorber layers, but would not work for CIGS at itsnormal processing temperatures. In addition the patent teaches the useof a clear conducting oxide (ZnO) as a barrier layer between the alloyand the absorber layer, as well as placing the alloy on a textured basemetal layer to increase the scattering angle. The ZnO layer providesconductivity and inhibits migration, but like all useful clearconducting oxides, it is an n-type semiconductor. When it is placedagainst the p-type absorber layer, a weak p-n junction is formed, whichacts to apply an undesirable small reverse electrical bias to the cell.The primary p-n junction must then overcome this back bias to causeuseful current to flow, thus degrading the net efficiency.

Iwasaki et al are on the right track, but there are two impediments tothe performance of their reflector. First, the ZnO barrier layer shouldnot be an n-type semiconductor; and secondly, alloys generally havepoorer conductivity and reflectivity than the pure metals. Among thetransition metal nitrides, borides, silicides, and carbides, severalhave high electrical conductivity; and additionally, they have highmelting temperatures and are relatively inert. A few have desirableoptical properties. The most optimum materials are the nitrides of someof the transition metals, and in particular titanium nitride (TiN),zirconium nitride (ZrN), and hafnium nitride (HfN). These nitrides havehigh melting points (about 3000 C for ZrN) and higher electricalconductivity than their parent metals; therefore, they do not act assemiconductors. Additionally, they have good optical properties;specifically, low indices of refraction similar to the noble metals.These properties make them very useful for forming an improved backcontact/reflecting layer in solar cells. All of the above mentionednitrides work well, but zirconium nitride has somewhat better opticaland electrical properties, and it is the one discussed herein asrepresentative of the entire class of metal nitrides.

FIG. 2 shows the computed reflectivity in air of 0.5 micron thick(opaque) films of molybdenum, niobium, nickel, copper, silver, aluminum,and zirconium nitride from 400 to 1200 nm. This spectral range coversthe principal region of the solar radiant output, which lies above aphoton energy of about 1 electron volt (ev). For a single junction solarcell a band gap of 1.4 to 1.5 ev is the optimum for highest efficiency,and in this region niobium and molybdenum, have reflection that isinferior to that of any of the other metals. The metallic nature ofzirconium nitride is evidenced by its relatively high reflectivity ascompared to molybdenum, niobium, and nickel. The reflectivity of a metalin air depends upon the optical indices of the air and the metal, whichof course vary with wavelength. The simple formula for the reflection atthe air/metal interface is:$R = \frac{\left( {n_{m} - n_{o}} \right)^{2} + k_{m}^{2}}{\left( {n_{m} + n_{o}} \right)^{2} + k_{m}^{2}}$where n_(o) is the refractive index of air (˜1) and n_(m) and k_(m) arethe refractive index and extinction coefficient of the metal. For ametal like silver the refractive index is much smaller than one, and theextinction coefficient is larger than one, so the k_(m) ² term dominatesand the reflection approaches 100% for thick films. In the case ofmolybdenum, niobium, and nickel both n and k are greater than one in thevisible spectral region, so their reflections work out to besubstantially less because of the (n_(m)−\+n_(o))² terms.

It happens that most semiconductors also have a refractive index ofabout 3, and this is particularly true for CIGS and CdTe, two of theleading contenders for thin-film solar cell absorbers. The formula forthe reflection suggests that the back reflection layer should not have nand k values near to 3. It seems that few if any in the industry havenoticed or discussed this potential problem with molybdenum inparticular. FIG. 3 shows the computed reflection of these metals at theinterface between the CIGS layer and the metal back conducting andreflecting layer, which is the way the layer actually functions in thesolar cell. Note that, as suggested above, the reflection of themolybdenum is greatly reduced from its value in air—by more than afactor of 2 in the most critical spectral regions. The reflections ofniobium and nickel fair somewhat better, but are also reducedsignificantly. The reflections of the other metals are not reduced asmuch because their refractive indices differ more markedly from thevalue 3. Nickel is a better reflector than molybdenum, and it would bemore economical; however, its tendency to diffuse is a potentialproblem, and since it is magnetic, it is more difficult to sputter thannon-magnetic metals. Zirconium nitride would be an excellent solution,with much better reflection than molybdenum, niobium, or nickel.However, the zirconium nitride would need to be about 1.5 microns thickto provide the same total electrical conductivity as 0.5 microns ofmolybdenum. It is possible to manufacture such a thick film with someeconomy using reactive sputtering; however, there is a better solution.

FIG. 4 shows the reflection of the metals in the previous two figureswhen a 15 nm thick barrier layer of zirconium nitride is placed betweenthe CIGS layer (or a CdTe layer) and the metal layer. The reflection ofthe molybdenum, niobium, and nickel are significantly improved, whilethe reflections of the other metals are slightly reduced. As thethickness of the zirconium nitride layer is further increased, thereflectance at the interface for all of the metals approaches that ofthe thick zirconium nitride shown in FIG. 7 (over 70%). In factcalculations predict that, at a zirconium nitride barrier layerthickness of approximately 100 nm (or 0.10 microns), the metal layerunderneath the zirconium nitride has little to no effect on thereflectance of light back through the CIGS layer—it becomes totallydominated by the properties of the zirconium nitride barrier layer.

As an example FIG. 5 shows the reflection at a wavelength of 800 nm formolybdenum and silver at the absorber/reflector interface as thethickness of the zirconium nitride layer varies from zero to 200 nm. Formolybdenum the reflection first increases sharply as the thickness ofthe zirconium nitride barrier layer increases, but it begins to roll offat a thickness of about 30 nm and changes very slowly after a thicknessof about 60 nm. At a thickness of 100 nm further change in thereflection is imperceptible. Reflection results for niobium and nickel(not shown) behave in a manner very similar to molybdenum. Thereflections start at a higher level than molybdenum, but they quicklyapproach the same limit. For silver the reflection starts out at highreflection (about 95%) and falls to that of thick zirconium nitride overabout the same thickness range as the case for molybdenum. In generalmetals that are poor reflectors need thicker zirconium nitride barrierlayers, and metals that are very good reflectors should have thinnerbarrier layers, i.e. just enough to do the job of protecting theabsorber/reflector interface. So a thin layer of ZrN acts like a metaland prevents the formation of an inverse p-n junction. It improves thereflection of the optically poor metals, and protects the CIGS layerfrom diffusion by the highly reflective metals. Since the opticalproperties are separated from the conductivity requirements of the backcontact layer, a wider range of choices for the base metal layer arepossible.

Accordingly, the present invention relates to a roll-to-roll depositionapparatus and method for producing an all sputtered thin-film CIGS solarcell, wherein the CIGS absorber layer is formed by co-deposition from apair of rectangular planar or cylindrical rotary magnetrons using directcurrent (DC) sputtering. The buffer layer of ZnS is RF sputtered from aconventional planar magnetron housed in a special chamber, thusreplacing the toxic CdS with a more benign material. The remaininglayers in the cell are formed by deposition from dual magnetronsutilizing DC and alternating current (AC) sputtering. Thus the cell ismanufactured in a single pass through a large modular vacuum depositionmachine with no wet processes or high temperature gas diffusionprocesses involved. The back contact/reflecting layer is improved by theaddition of a material not used in solar cells previously. In apreferred embodiment of the invention, the CIGS layer is deposited fromdual cylindrical rotary magnetrons, used in the configuration that isdescribed in U.S. Pat. No. 6,365,010 (which is incorporated herein byreference), in which one target contains copper and selenium while thesecond target contains indium, gallium, and selenium or indium,aluminum, and selenium.

A primary objective of the present invention is to provide a large-scalemanufacturing system for the economical production of thin-film CIGSsolar cells.

An additional objective of the invention is to provide a manufacturingprotocol for solar cells in which high temperature toxic gases and toxicwet chemical baths are eliminated from the process.

Another objective of the invention is to provide a manufacturing processfor CIGS solar cells, which significantly lowers their cost,specifically by improvements in the back contact/reflection layer andthe elimination of cadmium and the disposal of its toxic wastes.

A further objective of the invention is to provide an apparatus andmanufacturing process for CIGS solar cells, which significantlyincreases the size of substrate that can be used, including primarilycontinuous webs of material deposited in a special customized andmodularized roll-to-roll coating machine with increased capabilities andefficiencies.

The present invention is a method of manufacturing a solar cell thatincludes providing a substrate, depositing a conductive film on asurface of the substrate wherein the conductive film includes aplurality of discrete layers of conductive materials, depositing atleast one p-type semiconductor absorber layer on the conductive film,wherein the p-type semiconductor absorber layer includes a copper indiumdiselenide (CIS) based alloy material, depositing an n-typesemiconductor layer on the p-type semiconductor absorber layer to form ap-n junction, and depositing a transparent electrically conductive topcontact layer on the n-type semiconductor layer.

In another aspect of the present invention, a method of manufacturing asolar cell includes providing a substrate, depositing a conductive filmon a surface of the substrate, depositing at least one p-typesemiconductor absorber layer on the conductive film wherein the p-typesemiconductor absorber layer includes a copper indium diselenide (CIS)based alloy material, and wherein the deposition of the p-typesemiconductor absorber layer includes co-sputtering the CIS materialfrom a pair of conductive targets, depositing an n-type semiconductorlayer on the p-type semiconductor absorber layer to form a p-n junction,and depositing a transparent electrically conductive top contact layeron the n-type semiconductor layer.

In yet another aspect of the present invention, a method ofmanufacturing a solar cell includes providing a substrate, depositing aconductive film on a surface of the substrate, depositing at least onep-type semiconductor absorber layer on the conductive film wherein thep-type semiconductor absorber layer includes a copper indium diselenide(CIS) based alloy material and wherein the deposition of the p-typesemiconductor absorber layer includes reactively AC sputtering materialfrom a pair of identical conductive targets in a sputtering atmospherecomprising argon gas and hydrogen selenide gas, depositing an n-typesemiconductor layer on the p-type semiconductor absorber layer to form ap-n junction, and depositing a transparent electrically conductive topcontact layer on the n-type semiconductor layer.

In yet one more aspect of the present invention, a solar cell includes asubstrate, a conductive film disposed on a surface of the substratewherein the conductive film includes a plurality of discrete layers ofconductive materials, at least one p-type semiconductor absorber layerdisposed on the conductive film wherein the p-type semiconductorabsorber layer includes a copper indium diselenide (CIS) based alloymaterial, an n-type semiconductor layer disposed on the p-typesemiconductor absorber layer wherein the p-type semiconductor absorberlayer and the n-type semiconductor layer form a p-n junction, and atransparent electrically conductive top contact layer on the n-typesemiconductor layer.

In still yet one more aspect of the present invention, a vacuumsputtering apparatus includes an input module for paying out substratematerial from a roll of the substrate material, at least one processmodule for receiving the substrate material from the input module, andan output module. The process module includes a rotatable coating drumaround which the substrate material extends, a heater array for heatingthe coating drum, and one or more sputtering magnetrons each having amagnetron housing and a plurality of conductive sputtering targetsdisposed in the magnetron housing and facing the coating drum forsputtering material onto the substrate material. The output modulereceives the substrate material from the process module.

Other objects and features of the present invention will become apparentby a review of the specification, claims and appended figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the prior art structure of a basicCIGS solar cell.

FIG. 2 shows the computed reflection in air of metals commonlyconsidered useful as solar cell back contact layers. Included is a newclass of materials represented by zirconium nitride.

FIG. 3 shows the computed internal reflection at the interface between aCIGS absorber layer and the metals and zirconium nitride shown in FIG.2.

FIG. 4 shows the computed internal reflection at the interface between aCIGS absorber layer and the metals shown in FIG. 2 with a 15 nm thicklayer of zirconium nitride placed at the interface.

FIG. 5 shows the reflection at 800 nm at the absorber/reflectorinterface in a solar cell as a function of the thickness of a zirconiumnitride barrier layer.

FIG. 6 shows the structure of the basic solar cell of the presentinvention, wherein a thin zirconium nitride is inserted between the CIGSlayer and the back conducting/reflecting metal layer.

FIG. 7 shows an alternative structure for the solar cell of the presentinvention, wherein the back conducting/reflecting layer is improved withcopper and silver layers.

FIG. 8 shows schematically the co-sputtering of CIGS material fromconventional dual rectangular planar magnetrons.

FIG. 9 illustrates schematically the preferred embodiment of the DCco-sputtering of CIGS material from dual cylindrical rotary magnetrons.

FIG. 10 shows schematically an alternative method of using AC power toco-sputter the CIGS material.

FIG. 11 shows schematically an alternative AC reactive sputtering methodusing dual cylindrical rotary magnetrons with identical metal alloytargets to form the CIGS material.

FIG. 12 illustrates schematically the use of three sets of dualmagnetrons to increase the deposition rate and grade the composition ofthe CIGS layer to vary its band gap.

FIG. 13 shows the structure of a preferred embodiment of an improved allsputtered version of the basic solar cell of the present invention.

FIG. 14 shows a highly simplified schematic diagram of the side view ofa roll-to-roll modular sputtering machine used to manufacture the solarcell depicted in FIG. 13.

FIG. 15 shows a more detailed schematic diagram of a section of aprocess module with details of the construction of the coating drum andthe magnetron.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will now be described and compared with respect to theconventional prior art CIGS solar cell structure. The new cell structureand the manufacturing process will be detailed in relation to a modularroll-to-roll sputter deposition system designed specifically to providean optimum implementation of the process.

It should be noted that, as used herein, the terms “over” and “on” bothinclusively include “directly on” (no intermediate materials, elementsor space disposed therebetween) and “indirectly on” (intermediatematerials, elements or space disposed therebetween). For example,forming an element “on a substrate” can include forming the elementdirectly on the substrate with no intermediate materials/elementstherebetween, as well as forming the element indirectly on the substratewith one or more intermediate materials/elements therebetween.

FIG. 6 illustrates one of the simplest embodiments of a basic solar cellaccording to the present invention, which includes a zirconium nitridebarrier layer. The figure is similar to the conventional solar cellshown in FIG. 1 except for the added barrier layer 2 a of zirconiumnitride between the CIGS layer 3 and electrical contact layer 2. Assuggested above, electrical contact layer 2 can now be any of the metalsthat were discussed above or any economical metal with adequateconductivity. The alloys claimed by Iwasaki et al in '204 will worksince the zirconium nitride barrier layer will block diffusion whileretaining good reflectivity. Pure silver would give the optimum inperformance; however, it would be a relatively expensive solution.Aluminum is the cheapest good reflector, but its melting point isrelatively low (660 C) compared to the other metals, and it gettersoxygen from the background water vapor in a vacuum system, which lowersits conductivity.

An alternate embodiment of the solar cell of the present invention isshown in FIG. 7, where electrical contact layer 2 is made from copperinstead of molybdenum. Copper is relatively inexpensive and a very goodconductor. At about 0.2 microns thickness, it provides as muchelectrical conductivity as 0.5 microns of molybdenum. Layer 2 a is athin barrier layer of zirconium nitride that has a thickness in therange of approximately 10 to 20 nm. At this point the layer structure isthe same as that discussed in FIG. 6, and it could be used in this form,especially with CIGS because of its modest band gap. However, the lowerreflection shortward of about 600 nm (see FIG. 4) can be remedied by athin layer 2 b of silver (40 to 50 nm) deposited on top of the zirconiumnitride layer, and an additional barrier layer 2 c of zirconium nitridebetween the silver and the CIGS layers. With this structure the internalreflection at the silver/ZrN/CIGS interface is practicallyindistinguishable from the reflection curve labeled “silver” in FIG. 4,and the use of large amounts of the more expensive silver is avoided. Ifthe CIGS processing temperature could be significantly lowered from thecurrent value of about 550 C, then the intermediate barrier layer 2 abetween the copper and silver could be eliminated for temperatures belowwhich copper and silver would rapidly inter-diffuse. Alternatively, fora low enough processing temperature aluminum could be substituted forboth the copper and the silver. Of course, if the substrate were a metalfoil instead of glass, the base metal layer could be made thinner whileretaining the necessary reflectivity, since the metal foil would providemost of the conductivity.

The next layer to be described is the CIGS absorber. In this inventionthe preferred deposition method for the CIGS material is DC magnetronsputtering; however, AC reactive magnetron sputtering is also a viablealternative method diminished only by the added necessity of handlingsmall amounts of the toxic gas hydrogen selenide. Both methods utilizethe magnetron technology taught in '010 as the most desirable method;although, the invention may be practiced less effectively withconventional planar magnetrons. One reason DC magnetron sputtering hasnot been done with the CIGS material is that the electrical conductivityis too low because of its semiconductor nature. DC sputtering requiresmetal-like electrical conductivity as well as good thermal conductivityto allow high power for high deposition rates. Conceptually, oneimportant idea in this invention is to divide the CIGS material into twoparts, each part having properties that would permit the fabrication ofa conductive sputtering target. For the majority of semiconductors thatare serious candidates for use as absorber layers in solar cells thiswould be impossible, but the results of recent experiments demonstratethat it works for CIGS. After several failed attempts of variouscombinations, it was discovered that copper and selenium could becombined into a conductive matrix provided that the material wasprocessed properly. A homogeneous mixture of powders consisting ofapproximately two parts Se and one part Cu remains highly conductive ifit is cold pressed and annealed at a temperature somewhat below themelting point of Se (217° C.). Small samples made at 208 to 210° C. hadgood physical strength, and the electrical resistance was less than oneohm. When the annealing temperature is raised to about 400 C theresistance goes up by a factor of more than a million as might beexpected from the formation of CuSe₂. But the conductive properties ofthe lower temperature material are not easily reconciled with theexisting phase diagrams for the Cu—Se binary system. If chemicalreactions between the Cu and Se do not occur at the low annealingtemperature then the Se could be acting as a binder to hold a highlyconductive copper matrix together. For this to be the case, the Cu wouldhave to diffuse rapidly at the low annealing temperature, which isunlikely. The Cu₂Se phase is the only Cu/Se phase known to beconductive, so it probably forms although it does not appear to beconsistent with the phase diagram for this composition and temperature.However, since the material changes its appearance after annealing, itseems to favor a reaction having occurred. Similar experiments in whichthe Cu was replaced with In did not yield highly conductive matrices. Infact the resistance increased with indium content even at low annealingtemperatures. Since In and Se have low melting points, the observedresult might be expected, and unlike the Cu, it is consistent with theIn—Se phase diagram.

Regardless of the inconsistency with the phase diagram, the Cu/Se hasbeen made with the necessary properties for high rate DC magnetronsputtering. The target for the rest of the material must contain theindium and gallium needed to complete the CIGS structure. In and Ga arereadily melted together to form a low temperature solder which can bepoured or cast into a mold surrounding a backing or carrier tube to formthe target. Good mixing and rapid quenching is required to preventsegregation and formation of the low temperature eutectic. A moredesirable approach is to form the target by compressing metal powders,and in particular to include the gallium as gallium selenide (Ga₂Se₃).The target remains conductive and the low temperature eutectic isavoided. Additional Se may also be added and reacted with the In to formthe insulating In₂Se₃ phase, but as long as enough free In is left toform a conductive matrix, the target will sputter adequately. About halfof the In/Ga target can be Se, and remain conductive enough to sputtersince it takes three atoms of Se for every two atoms of In or Ga.Replacing the gallium with aluminum raises the eutectic melting pointsubstantially, without causing any further technical difficulties. Theinclusion of selenium in the In/Ga or In/Al target in addition to theselenium from the copper target provides an overpressure of seleniumduring the deposition process that is highly desirable.

Another advantage this target construction technology offers is a way todope the materials in many different and potentially beneficial ways.For example, it has been known for a long time that a very small amountof sodium (Na) added to the CIGS can improve its performance. Initially,it was noticed that cells made on soda lime glass had higher efficiencythan those made on other substrates, particularly stainless steel. Laterit was discovered that traces of Na from the glass were diffusing intothe CIGS during deposition. However, a way to add a small but controlledamount of Na easily for non-glass substrates has proved difficult. Withthe target forming method of this invention it is easy to introducetrace amounts (e.g. about 0.1%) of NaSe₂ into either the Cu/Se or theIn/Ga/Se to achieve the desired doping in the absorber layer.

The following description of sputtering CIGS material is made withrespect to a pair of sputtering targets: one comprising of Cu and Se,and the other In, Ga, and Se. The ratio of Cu to Se is approximately 1to 2, but may be varied to accommodate process variations andrequirements. The In to Ga ratio is varied to change the band gap, andit can range from In alone (band gap of 1 ev) to about 30% Ga (band gapof 1.3 ev). It should be noted that changes in the ratios of thematerials in each target as well as the additions of small levels ofdoping (as described above for Na) with other elements are considered tobe consistent with the basic invention.

Conventional DC rectangular planar magnetron co-sputtering of the CIGSmaterial is shown schematically in FIG. 8. The view is a cross-sectiontaken perpendicular to the long axes of the magnetrons. Elements 7represent the main bodies of the conventional magnetrons, which housethe magnetic assemblies (not shown) that form the sputtering “racetrack”and the means of cooling targets 8 and 9. The magnetrons are oriented sothat a line perpendicular to each target intersects at substrate 10,which is approximately 10 cm away. Each magnetron is powered by a DCpower supply 11, which is grounded to chamber wall/shield 12 that servesas the system anode. Alternatively, separate anodes (not shown)intimately associated with each magnetron could be provided as is commonin the art. Baffle 13 is placed between the magnetrons to help restrictmaterial sputtered from one source from depositing on, and reactingwith, that of the other source. The reacted material would be largelyinsulating and therefore undesirable since it builds up over time onareas of the planar target that are not sputtering. The baffle shouldnot protrude toward the substrate so far that the flux reaching thesubstrate is significantly reduced. If grounded, as indicated, it couldfunction as an anode or partial anode for each magnetron. All sputteringprocesses use a working gas, which is almost universally argon. Since itis inert, it may be introduced almost anywhere in the system. In FIGS. 8through 12 the argon injection location is not shown explicitly;however, injection at the rear or sides of the magnetron is conventionaland appropriate.

Still referring to FIG. 8, one of the targets, 8 for instance, comprisesthe conductive Cu/Se material, while target 9 comprises the conductiveIn/Ga/Se material. A substrate 10 is heated to a temperature of between400 and 600 C, and is transported past the magnetrons at a uniform rateas indicated by the arrow. Argon is introduced as the working gas at apressure of approximately 1 to 2 millitorr, and DC power is applied tosputter the materials. One power supply (11) is adjusted to achieve anacceptable sputtering rate for one of the two targets. The other powersupply is then adjusted until the reacted coating on the heatedsubstrate has the correct copper deficient composition. If the severalconstituents in each target have the same sputtering distributionpattern (although the two target patterns may differ from each other)then the correct composition will be achieved with power supplyadjustments alone. In general this may not be the case, in part becausethe individual elements may have different sputtering patterns. Thus,one constituent may be preferentially collected on nearby shielding,shifting the coating composition slightly from that expected from theoriginal target composition. Adjustments in the compositions of eachtarget by only few percent will correct the discrepancy, but the exactcomposition is dependent on many factors including machine geometry,sputtering pressure and sputtering power, so the correct compositionsmust be worked out for each unique machine setup. Once the compositionsare determined, they remain constant until there is a change in theprocess or system geometry. Such small variations in target compositionto accommodate system geometry are considered to fall within the scopeof this invention.

Process problems develop with the rectangular planar magnetronembodiment when the targets are sputtered for long periods of time, aswould be the case for a large-scale manufacturing operation. Asputtering groove 14 (dashed line) gradually forms defining the“racetrack” on each target as the deposition process proceeds. Thewell-known cosine distribution, which describes the local flux emissionpattern, is oriented perpendicular to the emitting surface. Therefore,the flux distribution at the substrate gradually changes as the targeterodes and the groove forms. If the patterns from the two magnetrons donot change in synchronism with each other, the composition of the CIGSmaterial deposited at the substrate will change with time, requiringadjustments to be determined and applied to the process almostcontinuously.

A second problem is that baffle 13 will not totally stop flux mixingbetween the targets over extended run times. This means that eventuallysignificant amounts of partially insulating reaction products will buildup on regions of the targets that are not being sputtered (i.e. at theedges of the “racetrack”). This can lead to arcing and defects in theCIGS film. Finally, the utilization of the target material ranges fromabout 25 to 40% for the planar targets, and they must be changed often,thus raising the manufacturing costs.

If cylindrical rotary magnetrons are substituted for the planarmagnetrons in FIG. 8, the setup becomes that shown in FIG. 9, wherecommon and similar elements are labeled with the same numerals. If theyare operated identically to that of the planar magnetron setup, theproblems associated with the planar magnetron embodiment are largelyeliminated. Since they rotate, a sputtering groove never forms. So thecomposition of the coating remains constant as the target material isconsumed, because the emission pattern of the flux remains fixed. Also,because of the rotation and subsequent continual target cleaning, therecan be no long term increasing build up of reacted material on thetargets for the same reasons that pertain to reactive sputtering asdetailed in '010. For this reason baffle 13 (shown dashed) is not asimportant in the rotary magnetron embodiment. If the diameter of therotary target is equal to the planar target width, and the targetmaterial is the same thickness, then the rotary target has over threetimes the initial inventory of material as the planar. And, because theutilization is more than double that of the planar, the rotary targetswill run more than six times as long as the planar targets before targetchanges are necessary. This is a significant cost saving factor forlarge-scale manufacturing.

Either the planar or the rotary magnetrons could be run in AC mode. Thisis illustrated in FIG. 10 for the rotary magnetrons, but the setup wouldapply as well to the planar magnetrons. The two DC power supplies 11 arereplaced by a single AC power supply 15. In order to vary the depositionrates between the targets to maintain a copper deficient filmcomposition, a variable impedance load 16 must be inserted into one ofthe legs of the AC supply. Since AC operation with dual magnetrons doesnot require a separate anode, the chamber wall/shields 12 no longer needto be grounded and neither does baffle 13. This alternative setup usingAC power when the conductive targets will support DC operation offerslittle advantage for the rotary magnetrons, but since the planarmagnetrons are not self-cleaning, it could offer some protection fromarcing in that setup.

As mentioned above AC reactive sputtering of the CIGS material is aviable alternative to DC sputtering if facility arrangements are made tohandle small amounts of hydrogen selenide gas, or other potentiallyuseful gases. FIG. 11 shows this setup for a pair of rotary magnetrons.It differs from that shown in FIG. 10 in a number of respects.

First, targets 8 and 9 are now identical, consisting of an alloy of themetals copper, indium, and gallium (or aluminum) selected to give aslightly copper deficient composition and a desired band gap. Basicallythe atomic ratio of copper to indium plus gallium or aluminum should beslightly less than one, with the ratio of indium to gallium or aluminumdetermining the band gap. The metal targets can be made usingconventional melting and casting techniques. Since the targets are nowidentical in composition, baffle 13 may also be eliminated. In additionto using argon as the conventional sputtering gas, hydrogen selenide gasis fed into the system near the substrate through, for example, nozzles17 to react with the sputtered metal atoms and form the CIGS material ina continuous process.

To date the best candidates for high efficiency thin-film solarabsorbers contain materials that must be made in complex structures, orthat form or use compounds and gases that are toxic. At the present timethere is one class of materials that show some promise for changing thissituation, and those are the nitrides of the IIIA elements aluminum,gallium, and indium. The nitrides of various mixtures of In/Ga and In/Aldisplay a range of band gaps that span the range of the solar spectrum.So far the techniques for making them as p-type semiconductors have notbeen perfected. Such an absorber system would be ideal for productionwith the rotary magnetrons of '010. The setup would be like that shownin FIG. 11 except the toxic reactive gas hydrogen selenide would bereplaced with harmless nitrogen. The transition metal nitride layer(i.e. ZrN) previously discussed is formed precisely in this way.

Since the CIGS layer (or other absorber layer) is relatively thick, thethroughput of the sputtering machine can be improved by using two ormore pairs of magnetrons to deposit the layer. Because the cost of themagnetrons is moderate compared to the overall cost of the vacuumsystem, the increased production rates more than offset the moderateincrease in the initial capital costs. For an in-line machine that coatsdiscrete substrates, throughput can also be increased by placingmagnetron sources on both sides of the machine, and coating twosubstrates on the same pass. The need to use multiple pairs ofmagnetrons to increase the rate of deposition of the CIGS layer presentsanother opportunity that is exploited in the present invention. This isdiscussed below using a representative example.

FIG. 12 illustrates schematically the CIGS deposition region within asputtering machine equipped with three pairs of rotary (shown) or planar(not shown) magnetrons. It could represent a region from an in-linemachine, or if arranged in an arc, a region from a roll-to-roll coaterwith a web substrate carried on a drum. With respect to the direction ofmotion of substrate 10 (indicated by the arrow), the first pair ofmagnetrons is 18, the second 19, and the third 20. In each pair ofmagnetrons, one of the targets is Cu/Se with a properly adjustedcomposition as discussed above. However, the second target in each groupwould be, for example, just In/Se for 18, In/Se with 15% Ga for 19, andIn/Se with 30% Ga for 20. In this way the Ga content of the CIGS layerwould be step-wise graded from bottom to top with little or no Ga in thebottom region and some maximum amount of Ga in the top region. This willgrade the band gap from about 1 ev at the bottom to about 1.3 ev nearthe top of the layer. Inverting the target sequence or coating in thereverse direction would invert the band gap grading. Some smoothing ofthe stepped boundary could be obtained by placing the magnetrons closeenough together to allow some overlap in their deposition patterns;however, thermal diffusion of the material will cause some grading atthe interface between regions in any event.

The advantage of being able to adjust the CIGS composition easily byusing multiple sets of targets is that the band gap of the CIGS can beengineered to optimize the efficiency of the cell. Conventional wisdomwould suggest forming the highest band gap regions at the top layer andthe lowest band gap regions at the bottom in the same order as that usedin multi-junction cells. However, in practice inverting this structurein a single junction cell generally leads to improved efficiency througha broadening of the voltage gradient across the absorber. Without Ga (orAl) in the CIGS the band gap is about 1 electron volt (ev), while theoptimum for the solar spectrum is about 1.4 to 1.5 ev. Replacing In with30% Ga raises the band gap to about 1.2 ev. Further additions of Gastart to lower cell efficiency. If Ga totally replaces In, the band gapcan exceed 1.6 ev. Aluminum raises the band gap faster than Ga, allowinga bandgap of 1.45 without exceeding 30%. Sulfur replacement of some ofthe selenium also raises the band gap, but less effectively than Ga.Many combinations are possible, and for a wide range of additivematerials the targets, when fabricated as described herein, remainconductive enough to co-sputter by DC methods. If p-type nitrides can beperfected, the magnetron targets would be fabricated with varying ratiosof In/Ga and In/Al to achieve similar graded band gaps, and it could beaccomplished by standard reactive AC sputtering with nitrogen asdescribed in '010.

The conventional plated CdS n-type window or buffer layer is not usedbecause of the toxicity and waste disposal problems associated withcadmium. A substitute material that has been shown to work about as wellis ZnS, as previously noted. This material can readily be made in thepresent invention by AC reactive sputtering from elemental zinc targetsused in the setup described in FIG. 11. In this case the reactive gasthat is injected through nozzles 17 is hydrogen sulfide instead ofhydrogen selenide. Since hydrogen sulfide is also a dangerous gas, it isnot the method of choice for depositing the layer. Since the layer isvery thin, it can be RF sputtered without negatively impacting themanufacturing rate. However, as stated before, conventional RFsputtering presents challenges in large-scale because of thenon-uniformity associated with variable machine geometry. The method ofRF sputtering described below will overcome the objection presented byvariable geometry. ZnSe could be RF sputtered using the same RFtechnique, but it is less desirable than ZnS because its band gap issmaller, although both materials have larger band gaps than CdS.

Since most transparent conducting oxides are n-type semiconductors, itis somewhat of a mystery that the conventional ZnO, being an n-typesemiconductor, cannot also be used as the window layer to make the p-njunction. All previous attempts to do this have failed to yield a cellwith high efficiency unless a plated CdS “buffer” layer is placed inbetween the absorber and the ZnO. Some studies have pointed to theformation of gallium oxide at the interface as being at least part ofthe problem, although indium oxide and selenium oxide could form aswell. Oxidation damage to the interface can be caused by energeticnegative oxygen ions from the sputtering plasma bombarding the CIGSsurface during the initial growth phase of the ZnO overcoat. Also, theenergetic ions may cause physical damage to the interface.

This interface damage to the p-n junction may be minimized or eliminatedwith the use of a very thin sacrificial layer of a pure metal placedover the CIGS layer before the transparent conductive overcoat isapplied. It is well known that zinc, cadmium, and mercury doping willchange CIGS from p to n-type, but only zinc is substantially free oftoxicity and waste disposal problems. If a thin layer of zinc is used,it can serve a dual role. First, it can diffuse into the CIGS layerdoping it to n-type, and therefore move the p-n junction away from theinterface forming a homojunction. Secondly, it can “take the brunt” ofthe negative ion bombardment converting to ZnO or ZnS in the process,thus reducing or eliminating damage to the CIGS interface. Damage to theinterface is not necessarily limited to high-energy oxygen ions. Bothsulfur and selenium form high-energy ions in a sputtering plasma in amanner similar to oxygen. Metals other than zinc might be used; however,they would form p-n heterojunctions. For example, thin layers of some ofthe transition metals would protect the CIGS from oxidation, but wouldnot move the p-n junction by diffusion into the CIGS. In particularzirconium will be converted to zirconium oxide, which is also an n-typesemiconductor, and it is one of the alternative materials mentioned byUllal, Zweibel, and von Roedern.

The use of a sacrificial layer as just described can help protect thep-n junction and maintain a higher voltage across the depletion region.It is useful because highly conductive ZnO will not support holestability as well as a less conductive n-type interface material. Forthis reason it has become common practice to use what is called“intrinsic” ZnO or i-ZnO as an initial thin overcoat for the CdS to helpmaintain the depletion zone in regions where the chemical bath platedCdS is marginal. This form of ZnO is made by adding more oxygen to theprocess to make a less conductive and more transparent form of thematerial. Of course the use of i-ZnO alone is damaging to the interfacebecause of the energetic oxygen ions. Therefore sacrificial metalliclayers can substitute for the conventional plated CdS as long as theoxide that is produced is an n-type semiconductor.

After the n-type layer is properly formed to create the p-n junction,the top transparent electrode layer is deposited. A transparent andconductive form of ZnO has been the conventional material used for thislayer, largely because it is less costly compared to materials likeindium tin oxide (ITO) that is widely used in the display industry. ZnOis both less conductive and less thermally stable than ITO; however, ZnOdoped with aluminum has the approximate performance of ITO whileretaining much of the cost advantage of ZnO. The required level ofaluminum doping to achieve this result is about 2 percent. Similaramounts of other dopants have been shown to work almost as well (see“New n-Type Transparent Conduction Oxides” by T. Minami in MRS Bulletin,August 2000). Currently, large scale sputtering of the ITO in thedisplay industry is accomplished primarily by using planar ceramictargets, which are conductive but expensive to fabricate. Control of thereactive process in large scale when metallic targets are used has metwith little success. A similar problem exists with the large-scalecontrol of the reactive process for depositing aluminum doped ZnO. Inprinciple this problem can be solved by using ceramic targets as is donewith the ITO, but the extra cost for the target fabrication offsets muchof the advantage gained from the cheaper materials. In this inventionthe rotary magnetrons as described in '010 allow the use of the cheapermetallic targets, and provide the necessary control of the reactiveprocess for large-scale implementation. The reactive sputtering setup isidentical to that described in FIG. 11 where targets 8 and 9 are bothmetallic zinc targets with the appropriate doping of aluminum. Inaddition to the usual argon sputtering gas, oxygen is supplied to thedeposition region through nozzles 17.

FIG. 13 shows the preferred all sputtered CIGS solar cell structure ofthe present invention. Layer 1 (the substrate) is a high temperaturemetal or polymer foil. Stainless steel, copper, and aluminum are thepreferred metal foils for terrestrial power production, while very thintitanium and polyimide are preferred foils for space power applications.Electrically conductive layers 2, 2 a, 2 b, and 2 c are Cu, ZrN, Ag andZrN, respectively, as previously described in FIG. 7. The CIGS in layer3 has a graded band gap created by changes in the composition ofsuccessive targets as shown and described in FIG. 12. The method ofdeposition may be either the DC co-sputtered film described in FIG. 9 orthe reactively sputtered film described in FIG. 11. Semiconductor layer4 is RF sputtered ZnS (or ZnSe) replacing the CdS of the conventionalcell. As an additional feature, layer 4 a may be included as asacrificial metal layer that becomes an n-type semiconductor uponsubsequent reaction during the deposition of the next layer (i.e. withoxygen, sulfur, or selenium). Layer 5 is the transparent top electrodeand comprises of reactively deposited aluminum doped ZnO to takeadvantage of the improvement in performance over the conventional ZnO.As previously explained, a very thin portion of the aluminum doped ZnOat the layer 4 interface may have a higher resistivity to improve thejunction voltage. Layer 6 is the optional anti-reflection (AR) film andis actually a multi-layer stack (not shown) designed to optimize thelight absorption in the cell. Such an AR stack would be used in a spacepower application where environmental degradation from weather is not anissue. For terrestrial applications the basic cells (layers 1 through 5)are laminated to a protective glass cover sheet in a sealed module (notshown), and the AR layer, if used, is applied to the outer surface ofthe glass cover instead of directly to the cell. One might additionallysputter current collection grid lines on the top conducting oxide, andif the substrate is a metal foil, a thin solder wetting layer (tin forexample) may be sputtered on the back.

A simplified schematic side view of a roll-to-roll modular sputteringmachine for making the improved solar cell of FIG. 13 is illustrated inFIG. 14. In the direction perpendicular to the view plane the machine issized to support substrates between about two and four feet wide. Thiswidth is not a fundamental equipment limit; rather, it recognizes thepractical difficulty of obtaining quality substrate material in widerrolls. The machine is equipped with an input, or load, module 21 a and asymmetrical output, or unload, module 21 b. Between the input and outputmodules are process modules 22 a, 22 b, and 22 c. The number of processmodules may be varied to match the requirements of the coating that isbeing produced. Each module has a means of pumping to provide therequired vacuum and to handle the flow of process gases during thecoating operation. The vacuum pumps are indicated schematically byelements 23 on the bottom of each module. A real module could have anumber of pumps placed at other locations selected to provide optimumpumping of process gases. High throughput turbomolecular pumps arepreferred for this application. The modules are connected together atslit valves 24, which contain very narrow low conductance isolationslots to prevent process gases from mixing between modules. These slotsmay be separately pumped if required to increase the isolation evenfurther. Alternatively, a single large chamber may be internallysegregated to effectively provide the module regions, but it thenbecomes much harder to add a module at a later time if process evolutionrequires it.

Each process module is equipped with a rotating coating drum 25 on whichweb substrate 26 is supported. Arrayed around each coating drum is a setof dual cylindrical rotary magnetron housings 27. Conventional planarmagnetrons could be substituted for the dual cylindrical rotarymagnetrons; however, efficiency would be reduced and the process wouldnot be as stable over long run times. The coating drum may be sizedlarger or smaller to accommodate a different number of magnetrons thanthe five illustrated in the drawing. Web substrate 26 is managedthroughout the machine by rollers 28. More guide rollers may be used ina real machine. Those shown here are the minimum needed to present acoherent explanation of the process. In an actual machine some rollersare bowed to spread the web, some move to provide web steering, someprovide web tension feedback to servo controllers, and others are mereidlers to run the web in desired positions. The input/output spools andthe coating drums are actively driven and controlled by feedback signalsto keep the web in constant tension throughout the machine. In addition,the input and output modules each contain a web splicing region 29 wherethe web can be cut and spliced to a leader or trailer section tofacilitate loading and unloading of the roll. Heater arrays 30 areplaced in locations where necessary to provide web heating dependingupon process requirements. These heaters are a matrix of hightemperature quartz lamps laid out across the width of the coating drum(and web). Infrared sensors provide a feedback signal to servo the lamppower and provide uniform heating across the drum. In addition coatingdrums 25 are equipped with an internal controllable flow of water orother fluid to provide web temperature regulation.

The input module accommodates the web substrate on a large spool 31,which is appropriate for metal foils (stainless steel, copper, etc.) toprevent the material from taking a set during storage. The output modulecontains a similar spool to take up the web. The pre-cleaned substrateweb first passes by heater array 30 in module 21 a, which provides atleast enough heat to remove surface adsorbed water. Subsequently, theweb can pass over roller 32, which can be a special roller configured asa cylindrical rotary magnetron. This allows the surface of electricallyconducting (metallic) webs to be continuously cleaned by DC, AC, or RFsputtering as it passes around the roller/magnetron. The sputtered webmaterial is caught on shield 33, which is periodically changed. Anotherroller/magnetron may be added (not shown) to clean the back surface ofthe web if required. Direct sputter cleaning of a conductive web willcause the same electrical bias to be present on the web throughout themachine, which, depending on the particular process involved, might beundesirable in other sections of the machine. The biasing can be avoidedby sputter cleaning with linear ion guns instead of magnetrons, or thecleaning could be accomplished in a separate smaller machine prior toloading into the large roll coater. Also, a corona glow dischargetreatment could be performed at this position without introducing anelectrical bias. If the web is polyimide material electrical biases arenot passed downstream through the system. However, polyimide containsexcessive amounts of water. For adhesion purposes and to limit the waterdesorption, a thin layer of metal (typically chromium or titanium) isroutinely added. This makes the surface conductive with similar issuesencountered with the metallic foil substrates.

Next the web passes into the first process module 22 a through valve 24and the low conductance isolation slots. The coating drum is maintainedat an appropriate process temperature by heater array 30. Following thedirection of drum rotation (arrow) the full stack of reflection layersbegins with the first two magnetrons depositing the base copper layer (2in FIG. 13). The next magnetron provides a thin ZrN layer, followed bythe thin silver layer and the final thin ZrN layer. For a CIGS absorberlayer the band gap is low enough that little is gained by the thinsilver and final thin ZrN layer. In this case the reflector may consistof just the base copper layer and the first ZrN layer. Future higherband gap materials could benefit from the extra silver and ZrN layers.

The web then passes into the next process module, 22 b, for depositionof the p-type graded CIGS layer. Heater array 30 maintains the drum andweb at the required process temperature. The first magnetron deposits alayer of copper indium diselenide while the next three magnetrons putdown layers with increasing amounts of gallium (or aluminum), thusincreasing and grading the band gap as previously described. The gradingmay be inverted by rearrangement of the same set of magnetrons. The lastmagnetron in the module deposits a thin layer of n-type ZnS (or ZnSe) byRF sputtering from a planar magnetron, or a sacrificial metallic layer,which becomes part of the top n-type layer and defines the p-n junction.

Next the web is transferred into the final process module, 22 c, whereagain heater array 30 maintains the appropriate process temperature. Thefirst magnetron deposits a thin layer of aluminum doped ZnO which has ahigher resistance to form and maintain the p-n junction in coordinationwith the previous layer. The remaining four magnetrons deposit arelatively thick, highly conductive and transparent aluminum doped ZnOlayer that completes the top electrode. Extra magnetron stations (notshown) could be added for sputtering grid lines using an endless beltmask rotating around the magnetrons. If an AR layer is to be placed ontop of the cell, the machine would have an additional process module(s)in which the appropriate layer stack would be deposited. The extramodules could also be equipped with moving, roll compatible, maskingtemplates to provide a metallic grid and bus bar for making electricalcontact to the top electrode. The extra modules and masking equipmentadds significantly to the cost of producing the cell, and may only bejustified for high value added applications like space power systems.

Finally, the web passes into output module 21 b, where it is wound ontothe take up spool. However, an additional operation can be performedhere, which is beneficial in the later processing of the cells intomodules. A dual cylindrical rotary magnetron 34 becomes the means topre-wet the back of the substrate foil with solder. Metallic tinprobably has the best properties of the available solder materials foruse with a stainless steel foil, but there are many solder formulationsthat will work. Pre-wetting may be unnecessary for a copper foil if itis kept clean. An ion gun sputter pre-cleaning of the back surface ofthe foil before the. solder sputtering may also be done in the outputmodule similar to that in the input module. In addition the webtemperature must be below the melting point of the pre-wetting solder(about 232 C for tin).

FIG. 15 shows a typical process module with an expanded sectionrevealing details of coating drum 25 and magnetron housing 27. Thecoating drum is constructed with a double wall defining a gap 35 throughwhich a cooling gas or liquid may be circulated to regulate thetemperature of the drum and web 26. The web is maintained in tightcontact against the outer surface of the drum. Magnetron housing 27consists of a local rectangular chamber 36 that contains rotarymagnetrons 37 and 38 and the associated mounting hardware (not shown).The entire housing can be located at a variable but uniform distance,represented by gap 39, from the surface of the coating drum and web.This variable gap allows control of the flow of the sputtering gasesfrom chamber 36 into the larger process module 22 a, which is vigorouslypumped. Thus a large pressure differential is maintained between thebackground pressure in rectangular chamber 36 and the process module (22a), and each magnetron is effectively isolated from its neighbors. Argonsputtering gas, indicated by the arrow, is fed into chamber 36 through aset of tubes 40 which are spaced uniformly along its length. Forreactive sputtering, the reactive gas (e.g. oxygen, nitrogen, hydrogensulfide, hydrogen selenide, etc.) is fed into chamber 36 through twosets of tubes 41, each set being equally spaced along its length.Internal baffles 42 create corridors which direct the reactive gas tothe substrate, yet prevent coating flux from changing the conductancepath of the gas with time, insuring a steady state process. This setupclosely resembles that disclosed by Chahroudi in U.S. Pat. No. 4,298,444issued Nov. 3, 1981, and is incorporated herein by reference.Rectangular chamber 36 has been referred to as a “mini” chamber withinthe larger vacuum chamber. The major improvements are that dualcylindrical rotary magnetrons are substituted for the single rectangularmagnetron of the prior art, and the method of injection of thesputtering gases has been improved.

Rectangular “mini” chamber 36 provides the key to the use of RFsputtering for the deposition of the ZnS (or ZnSe) buffer layer from asingle planar magnetron, as opposed to the rotary magnetrons that areillustrated. This chamber forms an isolated geometrically uniformstructure that in turn provides a uniform electrical environment for theRF sputtering. This allows the RF sputtering to proceed uniformly alongthe length of the magnetron. In addition, the chamber is protected fromcontamination from the other neighboring sputtering sources, so thatminor back sputtering from the chamber walls consists only of the ZnSmaterial. Thus the ZnS n-type layer is protected from extraneous dopingby foreign contaminants.

It is to be understood that the present invention is not limited to theembodiment(s) described above and illustrated herein, but encompassesany and all variations falling within the scope of the appended claims.For example, as is apparent from the claims and specification, not allmethod steps need be performed in the exact order illustrated orclaimed, but rather in any order that allows the proper formation of thesolar cells of the present invention.

1. A solar cell, comprising: a substrate; a conductive film disposed ona surface of the substrate, wherein the conductive film includes aplurality of discrete layers of conductive materials, wherein thediscrete layers of conductive materials include: at least one metalliclayer of material selected from one or more groups comprising copper,silver, aluminum, molybdenum, and niobium; and at least one barrierlayer made substantially of a transition metal nitride material; atleast one p-type semiconductor absorber layer disposed on the conductivefilm, wherein the p-type semiconductor absorber layer includes a copperindium diselenide (CIS) based alloy material; an n-type semiconductorlayer disposed on the p-type semiconductor absorber layer, wherein thep-type semiconductor absorber layer and the n-type semiconductor layerform a p-n junction; and a transparent electrically conductive topcontact layer on the n-type semiconductor layer.
 2. The solar cell ofclaim 1, wherein the transition metal nitride material is selected fromone or more groups comprising titanium nitride, zirconium nitride, andhafnium nitride.
 3. The solar cell of claim 1, wherein the barrier layercomprises substantially of zirconium nitride.
 4. The solar cell of claim1, wherein the discrete layers of conductive materials include: aplurality of metallic layers of material each selected from one or moregroups comprising copper, silver, aluminum, molybdenum, and niobium; anda plurality of barrier layers each of a transition metal nitridematerial.
 5. The solar cell of claim 4, wherein the barrier layers areeach selected from one or more groups comprising titanium nitride,zirconium nitride, and hafnium nitride.
 6. The solar cell of claim 4,wherein the barrier layers each comprises zirconium nitride.
 7. Thesolar cell of claim 1, wherein the substrate comprises thin metallicfoil.
 8. The solar cell of claim 7, wherein the thin metallic foil isselected from one or more groups comprising stainless steel, copper, andaluminum.
 9. The solar cell of claim 1, wherein the p-type semiconductorabsorber layer has a graded bandgap.
 10. A solar cell, comprising: asubstrate; a conductive film disposed on a surface of the substrate,wherein the conductive film includes a plurality of discrete layers ofconductive materials, wherein the discrete layers of conductivematerials include: a first layer of copper; a second layer of silver;and a plurality of barrier layers each a transition metal nitridematerial; at least one p-type semiconductor absorber layer disposed onthe conductive film, wherein the p-type semiconductor absorber layerincludes a copper indium diselenide (CIS) based alloy material; ann-type semiconductor layer disposed on the p-type semiconductor absorberlayer, wherein the p-type semiconductor absorber layer and the n-typesemiconductor layer form a p-n junction; and a transparent electricallyconductive top contact layer on the n-type semiconductor layer.
 11. Asolar cell comprising: a substrate; a conductive film disposed on asurface of the substrate, wherein the conductive film includes aplurality of discrete layers of conductive materials; at least onep-type semiconductor absorber layer disposed on the conductive film,wherein the p-type semiconductor absorber layer includes a copper indiumdiselenide (CIS) based alloy material; an n-type semiconductor layerdisposed on the p-type semiconductor absorber layer, wherein the p-typesemiconductor absorber layer and the n-type semiconductor layer form ap-n junction; a transparent electrically conductive top contact layer onthe n-type semiconductor layer, and a layer of metallic materialdisposed between the p-type semiconductor absorber layer and the n-typesemiconductor layer.
 12. The solar cell of claim 11, wherein the layerof metallic material comprises zinc.